CFLs contain a very small amount of mercury sealed within the glass tubing – an average of 4 milligrams (mg). By comparison, older thermometers contain about 500 milligrams of mercury – an amount equal to the mercury in 125 CFLs. Mercury is an essential part of CFLs; it allows the bulb to be an efficient light source. No mercury is released when the bulbs are intact (not broken) or in use.  Most makers of light bulbs have reduced mercury in their fluorescent lighting products. Thanks to technology advances and a commitment from members of the National Electrical Manufacturers Association, the average mercury content in CFLs has dropped at least 20 percent or more in the past several years. Some manufacturers have even made further reductions, dropping mercury content to 1 mg per light bulb.

What are mercury emissions caused by humans? EPA estimates the U.S. is responsible for the release of 103 metric tons of mercury emissions each year. More than half of these emissions come from coal-fired electrical power. Mercury released into the air is the main way that mercury gets into water and bio-accumulates in fish. (Eating fish contaminated with mercury is the main way for humans to be exposed.)  Most mercury vapor inside fluorescent light bulbs becomes bound to the inside of the light bulb as it is used. EPA estimates that the rest of the mercury within a CFL – about 11 percent – is released into air or water when it is sent to a landfill, assuming the light bulb is broken. Therefore, if all 272 million CFLs sold in 2009 were sent to a landfill (versus recycled, as a worst case) – they would add 0.12 metric tons, or 0.12 percent, to U.S. mercury
emissions caused by humans
http://www.energystar.gov/ia/partners/promotions/change_light/downloads/Fact_Sheet_Mercury.pdf